How do I balance all my children's needs with limited time and one me?

My oldest child has been involved in a number of activities lately that have required a big time commitment from our whole family, and next week she is about to start another! I'm so happy that she's found some activities that she really cares about, but her sister seems to be between interests and is feeling a little sad. She has some activities starting up in about a month, but for a kid that's a long time to wait! I feel guilty about spending more time on the older child's needs right now, even though I know there have been times that my time was dominated by the younger child.

How do you all balance the needs of your various children with your limited time?